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Engineering Calculators

Reheat/Heating Sizing Calculator

Instructions: 

  1. Review the methodology to make sure it aligns with your project's requirements.Select US or Metric Units

  2. Enter the Reheat Coil Airflow, CFM [l/s]

  3. Enter the Leaving Reheat Coil Temperature, oF [oC]

  4. Enter the Entering Reheat Coil Temperature, oF [oC]

  5. The results are displayed in the Heating Coil Capacity Table

Note: for Reheat Coil Sizing Calculations, the Leaving Coil Temperature is customarily set to the room heating setpoint, and the Entering Coil Temperature is customarily set to the cooling coil leaving air temperature.

Heating coil capacity

Btu/h

Also calculated
Airflow unit
Leaving temperature unit
Entering temperature unit

Heating coil capacity

kWWatts Heating coil capacity Heating coil capacity, kW Heating coil capacity, Watts

For reheat coil sizing the leaving coil temperature is customarily the room heating setpoint and the entering coil temperature the cooling coil leaving air temperature, so the capacity comes out negative to denote heating.

Constants are the original workbook's: 1.0882 Btu/h per cfm per °F, and 3,412 Btu/h per kW. The published method text rounds the kilowatt conversion to 3,412.14163 instead, a difference of 0.004%.

Methodology and Equations:

In the context of air conditioning, "reheat" refers to a process that involves heating the air after the air conditioning system has cooled it. The purpose of reheating is to control the temperature and humidity levels in a conditioned space more precisely. In the reheat process, after the air has been cooled, it passes through a reheat coil, where it is reheated to a desired temperature and humidity level. The reheat coil is typically equipped with electric heaters or hot water coils. By adding heat back into the air, the system can achieve a more comfortable humidity level while maintaining the desired temperature.

Reheat is particularly useful in humid climates or situations where precise control over humidity levels is required, such as in laboratories, data centers, or specific industrial processes. 

Heating Coil Capacity = 1.0882 x Q x (TEntering -  TLeaving)

where:Q = Aiflow, CFM [l/s]

TEntering = Entering Coil Temperature, oF [oC]

TLeaving = Leaving Coil Temperature, oF [oC]​​

Example: Calculate the necessary reheating coil capacity for a 7.5-ton (3,000 CFM).  The room heating set point is 70oF, and the cooling coil leaving air temperature is 52oF.

To reiterate the note in the instructions, "for Reheat Coil Sizing Calculations, the Leaving Coil Temperature is customarily set to the room heating setpoint, and the Entering Coil Temperature is customarily set to the cooling coil leaving air temperature."

Reheat coil capacity = 1.0882 x 3,000 CFM x (52oF - 70oF) = -58,762.80 BTU/h (where the negative value denotes heating)

This can be converted to kilowatts using the formula: 1kW = 3,412.14163 BTU/h

= -58,762.80 BTU/h x 1kW / 3,412.14163 BTU/h = -17.22 kW (where the negative value denotes heating)

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